This invention relates to the protection of sensitive electronics components from electrostatic discharge (ESD) and is more particularly directed to a barrier that shields internal components of a device from ESD or other high voltage stray charge, and conducts away harmlessly any electrical charge that may find its way into the device.
Modern sophisticated electronic circuits, and particularly integrated solid state circuits based on metal-oxide-semiconductor (MOS) technology, is highly sensitive to electrostatic charge. Great care must be observed in circuit design and construction techniques when MOS circuitry is employed. Even such a simple gesture as touching the MOS integrated circuit package, when protective cautions are not observed, can cause an ESD spark to enter the device and destroy it.
After the circuitry has been installed, the need for ESD protection continues as electrostatic charge build up can occur during use of the electronic instrument. This charge can leak inside the housing of the instrument, so care must be taken to prevent it from reaching and destroying the high-voltage-sensitive electronic components.
In hand-held devices, especially those with plastic cases, ESD is an especial concern. Handling the device, sliding it along a table, or simply picking it up for use can generate electrostatic voltages of 25 kilovolts or more. While the case itself generally has a high electrical strength, there are leakage zones particularly along seams where the lid or cover, that provides access to the electronics, is fitted onto the main housing.
A number of approaches to ESD protection now exist, but these each have significant drawbacks and disadvantages, because they cannot reliably direct the inevitable leakage ESD away from sensitive internal components. These approaches include barriers, laminates, and conductive coatings.
Many high-voltage materials exist that serve as ESD barriers. These include Mylar, Valox, Lexan, Ultem, Nomex, and Kapton, among others. However, a complete barrier is impossible to create for thin materials and films. Therefore these materials require a substantial thickness, i.e. 10 mils or more, for 25 kilovolts of protection. These barriers also permit the electrostatic charge to continue to accumulate, until the voltage exceeds the strength of the barrier.
Laminate barriers employ a film of plastic material and a metal foil layer, and are frequently used in ESD protection or to control electromagnetic interference (EMI). The conductive side is disposed to the outside, i.e. facing the inside of the case, and the insulating side faces inward. These laminate barriers are intended to conduct ESD that enters the case to some grounding means. Unfortunately, because the conductive side extends over the entire shield, the ESD path can come in close proximity to the sensitive electronic components. This can lead to destruction of the very elements that the shield is intended to protect.
Conductive paints and inks are often employed to coat electronic devices for control of EMI, but they may also be employed for ESD protection. These are typically employed by coating the entire case with the conductive paint. As with the laminates, the pathway is not selective, and charge can be carried into proximity to the sensitive components.
Inks are used for EMI control and have been used in circuit board construction, but have not been printed onto a case or onto an insulating barrier.